Special dispatching communication system is a communication mode that appeared long ago, which is mainly used for special command, dispatching communication etc. The special dispatching communication system developed from the forms of one-to-one interphone, same-frequency simplex networking, different-frequency simplex (Bi-directional) networking to single channel one-call-hundreds-response and then selective calling system, and until the multi-channel automatic dialing system. In recent decade, the special dispatching communication system has developed to a higher level, and becomes a dispatching system that is shared by multi-channel users, being called the group communication system. One group system needs to have at least two functions, one is group calling, and the other is the function that the user can talk by pushing the “enter” key, without getting response from other users.
According to the technical mechanism adopted by the system, the group communication system can be divided into analog group communication system and digital group communication system. Currently, two kinds of widely used digital group communication systems of TDMA standard are iDEN (integrated Digital Enhanced Network) digital group communication system and land trunk wireless TETRA (Terrestial trunked Radio) communication system.
Both the above two realizations of trunk communication adopt the second generation digital communication technology, and use the TDMA mode; But nowadays, with the increasing sophistication of the third generation communication technology, the TDMA standard mechanism has such shortcomings as not being able to evolve to 3G (the third generation communication) freely, limited capacity, low handoff success rate and low data throughput. TDMA now is unable to satisfy the increasingly high requirements of communication.
CDMA (Code Division Multiple Access) system has broadband spreading frequency air interface, and already has the method of realizing trunk service in the CDMA2000 1x system. CDMA2000 HRPD (High Rate Packet Data) is based on data service, and it provides packet data service of high-rate. If CDMA2000 HRPD technology is combined with the group communication, the combination can not only exert the advantages of CDMA2000 HRPD technology, but can also satisfy the trunk service request of users, such as cluster-calling, group-calling and so on. But the existing HRPD system is only aimed at the needs of the trunk service, it is not capable to support the requirements of trunk communication function yet.